In gas turbine engines, energy is added to the air through the processes of compression and combustion, while energy is extracted by means of a turbine. In a turbofan engine, compression is accomplished sequentially through a fan and thereafter through a low-pressure compressor and high-pressure compressor, with the fan and low-pressure compressor being driven by a low-pressure turbine and the high-pressure compressor being driven by a high-pressure turbine through concentric shaft connections. Combustion occurs between the high-pressure compressor and the high-pressure turbine. Since the energy available to the turbines far exceeds that required to maintain the compression process, the excess energy is exhausted as high velocity gases through one or more nozzles at the rear of the engine to produce thrust by the reaction principle.
Typically, turbines within an engine are comprised of fixed geometries which are designed to provide balanced performance across a wide engine operating range. By introducing to a high-pressure and/or low-pressure turbine the ability to vary the turbine inlet flow area, the turbine can be adjusted to achieve optimal engine performance at multiple engine operating points. One metric of performance for the engine may be maximum achievable thrust. The level of engine thrust may be increased by allowing the high-pressure turbine to accept more flow for a given combustor exit temperature by increasing the high-pressure turbine inlet area, as governed by the flow area of the first row of high-pressure turbine vanes. Another metric of performance may be minimized fuel consumption. It is characteristic of some of these variable cycle engines that both the high and low pressure turbines contain mechanisms to allow for variable turbine inlet flow areas.
Various designs for providing a variable turbine inlet flow area using rotatable vanes have been proposed, but improvements are still needed in the art.